Question

I have something here that is really catching me off guard.

I have an ObservableCollection of T that is filled with items. I also have an event handler attached to the CollectionChanged event.

When you Clear the collection it causes an CollectionChanged event with e.Action set to NotifyCollectionChangedAction.Reset. Ok, that’s normal. But what is weird is that neither e.OldItems or e.NewItems has anything in it. I would expect e.OldItems to be filled with all items that were removed from the collection.

Has anyone else seen this? And if so, how have they gotten around it?

Some background: I am using the CollectionChanged event to attach and detach from another event and thus if I don’t get any items in e.OldItems … I won’t be able to detach from that event.

CLARIFICATION: I do know that the documentation doesn’t outright state that it has to behave this way. But for every other action, it is notifying me of what it has done. So, my assumption is that it would tell me … in the case of Clear/Reset as well.

Below is the sample code if you wish to reproduce it yourself. First off the xaml:

<Window     x:Class='ObservableCollection.Window1'     xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'     xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'     Title='Window1'     Height='300'     Width='300' >     <StackPanel>         <Button x:Name='addButton' Content='Add' Width='100' Height='25' Margin='10' Click='addButton_Click'/>         <Button x:Name='moveButton' Content='Move' Width='100' Height='25' Margin='10' Click='moveButton_Click'/>         <Button x:Name='removeButton' Content='Remove' Width='100' Height='25' Margin='10' Click='removeButton_Click'/>         <Button x:Name='replaceButton' Content='Replace' Width='100' Height='25' Margin='10' Click='replaceButton_Click'/>         <Button x:Name='resetButton' Content='Reset' Width='100' Height='25' Margin='10' Click='resetButton_Click'/>     </StackPanel> </Window> 

Next, the code behind:

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Windows; using System.Windows.Controls; using System.Windows.Data; using System.Windows.Documents; using System.Windows.Input; using System.Windows.Media; using System.Windows.Media.Imaging; using System.Windows.Navigation; using System.Windows.Shapes; using System.Collections.ObjectModel;  namespace ObservableCollection {     /// <summary>     /// Interaction logic for Window1.xaml     /// </summary>     public partial class Window1 : Window     {         public Window1()         {             InitializeComponent();             _integerObservableCollection.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(_integerObservableCollection_CollectionChanged);         }          private void _integerObservableCollection_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)         {             switch (e.Action)             {                 case System.Collections.Specialized.NotifyCollectionChangedAction.Add:                     break;                 case System.Collections.Specialized.NotifyCollectionChangedAction.Move:                     break;                 case System.Collections.Specialized.NotifyCollectionChangedAction.Remove:                     break;                 case System.Collections.Specialized.NotifyCollectionChangedAction.Replace:                     break;                 case System.Collections.Specialized.NotifyCollectionChangedAction.Reset:                     break;                 default:                     break;             }         }          private void addButton_Click(object sender, RoutedEventArgs e)         {             _integerObservableCollection.Add(25);         }          private void moveButton_Click(object sender, RoutedEventArgs e)         {             _integerObservableCollection.Move(0, 19);         }          private void removeButton_Click(object sender, RoutedEventArgs e)         {             _integerObservableCollection.RemoveAt(0);         }          private void replaceButton_Click(object sender, RoutedEventArgs e)         {             _integerObservableCollection[0] = 50;         }          private void resetButton_Click(object sender, RoutedEventArgs e)         {             _integerObservableCollection.Clear();         }          private ObservableCollection<int> _integerObservableCollection = new ObservableCollection<int> { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 };     } } 

Answer 1

Ok, even though I still wish that ObservableCollection behaved as I wished … the code below is what I ended up doing. Basically, I created a new collection of T called TrulyObservableCollection and overrided the ClearItems method which I then used to raise a Clearing event.

In the code that uses this TrulyObservableCollection, I use this Clearing event to loop through the items that are still in the collection at that point to do the detach on the event that I was wishing to detach from.

Hope this approach helps someone else as well.

public class TrulyObservableCollection<T> : ObservableCollection<T> {     public event EventHandler<EventArgs> Clearing;     protected virtual void OnClearing(EventArgs e)     {         if (Clearing != null)             Clearing(this, e);     }      protected override void ClearItems()     {         OnClearing(EventArgs.Empty);         base.ClearItems();     } }